NORTH-WEST OF ENGLAND AND NORTH WALES. 127 



have clay laid upon it as the bottom further subsided. Or in the direc- 

 tion of the greatest rush of the tide shingle might form, as is the case 

 now in places in the bed of the Irish Sea and English Channel*. 



Bearing these facts in view, let us see if we can apply them to an 

 explanation of the peculiarities of the deposits I have detailed, 

 The first-formed deposits were undoubtedly the gully gravels and 

 sands ; the next were the beds of gravel, current-bedded, lying at 

 the base of the Boulder-clay. As the land further subsided the 

 lower clay began to form, and with it became mixed the red sand 

 washed from the previously degraded Triassic rocks — the mixture of 

 materials being calculated to form a hard, short, red clay. On still 

 greater subsidence these clays would be brought within the reach 

 of cross tidal currents, creating those lines of erosion often mis- 

 taken for lines of unconformity representing geological subdivisions ; 

 and on this a greater or less bed of sand might or might not be 

 deposited. The deposition of more plastic clays would then set in ; 

 but there might be intermediate beds of sand still laid down. At a 

 certain depth of immersion the deposits would reach their maximum, 

 one element being the extent and nature of the land still unsub- 

 merged. But while this submergence was still going on, part of the 

 deposits would be washed in by the sea landwards, to be again 

 redistributed as they came successively under the influence of the 

 waves and tide. When the submergence reached its extreme 

 limit the amount of sediment contributed by the land to the ocean 

 would reach a minimum. As the land rose again out of the waters, 

 something of a similar kind would happen in inverse order, only the 

 rivers would reexcavate the drift which had been deposited in 

 them. 



I have already pointed out that the character of each basin is 

 influenced most profoundly by the nature of the rocks within it, so 

 that in different areas or basins diverse beds will mark the same 

 stages of submergence. If I have succeeded in establishing this 

 point, it follows as a natural consequence that the bulk of the drift- 

 deposits of these low-level plains have been formed when the seas 

 were comparatively shallow, probably at depths of from 100 to 

 300 feet. The shallower the sea the more local the deposits ; for 

 as the water-partings of the basins became submerged the nature 

 of the deposits would to a larger extent be due to the mechanical 

 distribution of the tides and waves. 



It would occupy too much time to attempt to explain each bed on 

 these principles ; but I am fully satisfied they are susceptible of it. 

 If my explanation be correct, it is evident that the successions of 

 beds are local phenomena, and that no geological subdivisions can 

 be founded upon them. 



Mountain- and Hill-Drift. 

 I have before remarked that the distinguishing feature of our 



* " Tidal Action as a Geological Cause," Proc. of Liverpool Geol. Soc, Session 

 1873-74. 



